Motor vehicle engines, and more particularly diesel engines, emit pollutant particles such as nitrogen oxides (NOx). To minimize the emission of such pollutants, the exhaust line of the engine generally includes a NOx trap to trap and reduce the NOx emitted by the engine.
The NOx trap typically has two operating states, namely a NOx storage state and a NOx reduction state, that are controlled as a function in particular of the richness of the exhaust gases entering the trap and the temperature of the trap.
Since the storage capacity of the trap is limited, it is necessary to regenerate the trap, i.e. to reduce the NOx stored therein, regularly and/or periodically.
To this end, the engine is generally associated with a system for supervising the operation of the NOx trap that controls the operation of the engine as a function of information such as the temperature of the trap, the operating point of the engine or the mass of NOx stored in the NOx trap, for example.
As a function of the above information, the supervisory system generally varies the injection of fuel into the cylinders of the engine to vary the richness and the temperature of the exhaust gases.
Some of the information used by the supervisory system is simple to acquire, for example by means of sensors. However, other information, and more particularly the mass of NOx stored in the NOx trap, is difficult to measure and has to be estimated on the basis of a model of the operation of the NOx trap.
This kind of estimation conventionally necessitates a considerable amount of calculation, with the result that it is difficult to implement on a motor vehicle unless a microcontroller of considerable computation capacity is used.
Using a model of the operation of the NOx trap may yield an inaccurate estimate if the conditions of operation of the NOx trap are not compatible with the characteristics of the model, which is conventionally based on a predefined range of operation of the NOx trap; thus operation of the NOx trap outside this range can lead to an inaccurate estimate of the mass stored in the trap.